Method of and apparatus for heating of liquids

ABSTRACT

A gaseaous combustible mixture, having a temperature lower than its ignition temperature, is passed with a speed exceeding its rate of flame propagation through a gas permeable layer of an radiation substance adapted to produce a thermal condition for radiating substantially the entire thermal energy, released at its surface, in the wave-length range of 0.5 - 6 micrometers within the active substance layer on. The apparatus comprises a reaction space filled with said substance and a supply system for the combustible mixture.

United States ateiit 1191 1111 3,777,717

Mach et a1. Dec. 11, 1973 [5 METHOD OF AND APPARATUS FOR 3,168,587 2/1965 HEATHNG 0F LlQUlDS 1,216,096 2/1917 2,087,031 7/1937 [75] Inventors: Jan Mach; Vaclav Rybar, both of 3,0 7,127 12 1962 Praha, Czechoslovakia 3,155,627 11/1964 3,421,859 1/1969 Kruggel 431/328 X [73] Assignee: llutnidruhovyroba,generalni d l P h l N ta a Czechos ovakla Primary Examiner-Kenneth W. Sprague 1 Flledl 1972 Attorney-Arthur O. Klein [21] Appl. No.: 239,433

[57] ABSTRACT [30] Foreign Application Priority Data A gaseaous combustible mixture, having a tempera- Apr. 12, 1971 Czechoslovakia 2377/71 .1 lower than its ignition temperature, is Passed with Nov.9, 1971 Czechoslovakia 7835/71 3 Speed exceeding its fate of flame Propagation through a gas permeable layer of an radiation sub- 52 us. c1. 122/156, 122/367 PF stance adapted to produce a thermal condition for 511 1m. (:1. .1221 13/02 dieting Substantially the entire thermal y.

[58] Field of Search 122/367 PF, 156; leased at its surface, in the wave-length range 0f 431/328; 52 5 6 micrometers within the active substance layer on. The apparatus comprises a reaction space filled with 5 References Cited said substance and a supply system for the combusti- UNITED STATES PATENTS 2,048,446 7/1936 Hays 122/367 9 Claims, 5 Drawing Figures METHOD OF AND APPARATUS FOR HEATING OF LIQUIDS BACKGROUND OF THE INVENTION The present invention relates to a method of heating liquids, for example in heating systems, distillation plants, evaporation processes, thermal decomposition or synthesis of liquids and relates also to an apparatus for effecting this method.

Known appliances for heating liquids comprise, for instance, boilers for heating or evaporating water for heating purposes, further all types of heat exchangers, in which liquids, such as water, crude oil, various solutions and the like are heated by heat obtained from the combustion of gaseous or liquid fuels. Such boilers or the like are usually equipped with a combustion space of large dimensions, in which the chemically bonded heat of the gaseous or liquid fuel is released during its combustion. The total heat transmission in such boilers is effected mainly by convection of the combustion gases and, to a lesser degree only, by radiation of the flame and of the three-atomic gas components. Considering that in the combustion of gaseous fuels and evaporated light liquid fuels the radiation of the flame and of the three-atomic components of the combustion products is slight, due to their relatively low temperature, a substantial part of the released heat is transmitted to the metal walls of the boiler by convection of the combustion gases. In the operation of these boilers such heat transmission results in a reduced output (kcal/m .h), considerable weight of the apparatus, increased heating surfaces (m and undesirable increase of the water content in the boiler (l).

Gas combustion in a ceramic layer has been suggested as long ago as in 1913, but has never led to any practical results and since then no appreciable experiments have been carried out in that direction. Failure was due to the fact, that the unsuitable combustion process, as applied, resulted in a specific output of the apparatus not exceeding the usual output of present classical boilers used in power engineering.

These facts are well known and have been described in a number of publications.

SUMMARY OF THE INVENTION The'present invention aims at removing the above mentioned disadvantages. The invention relates to a method of heating liquids by introducing into a reaction space a homogenous mixture of fuel and oxidising agent, in particular gas and air, having a lower temperature than its ignitiontemperature, and with a speed exceeding its rate of flame propagation, with the result that a flameless combustion is achieved.

The essence of the invention consists there in that the mixture of fuel and oxidising agent is passed in the reaction space through a gas permeable layer of an active substance, sometimes referred to herein as a radiation substance" adapted to produce a thermal condition for radiating substantially the entire thermal energy, released at its surface, in the wavelength range of 0.5 to

' 6 micrometers.

According'to further feature of the invention the pyrometric (operative) temperature in that part of the gas permeable layer of radiation substance,-in which the combustion is effected, is kept at a value at which a practically complete heat transmission by radiation occurs already in the reaction space.

The invention relates further to an apparatusfor carrying out said method, which apparatus comprises a hollow peripheral jacket holding the medium to be heated, and equipped with a mixing device for the fuel and oxidising agent. i

The apparatus according to the invention comprises a reaction space, filled with the above mentioned gas permeable layer of the radiation substance, as well as a supply system for the combustible mixture, arranged between said mixing device and the active substance in the reaction space.

The active substance contained in the reaction space, or its surface must be able to produce such thermal conditions for the combustion, that the entire thermal energy, released by combustion of the fuel, is directly changed into radiation energy and, substantially in its entirely, is transmitted to heat exchange surfaces in the most effective range of short wavelengths and in this way simultaneously prevents heat to be transmitted by convection. For this purpose the reaction space is filled with said gas permeable layer of active substance in the form of grains or shaped so as to provide therein a system of small chambers, channels, grids or cavities of various shape. The substance must be highly heatresistant and show a high emissive power in the required wavelength range. Preferably used in a substance capable of selective radiation, preponderantlyin the selected part of the spectrum. v

Such a substance is for example zirconium silicate but suitable are also some metals (such as tungsten or tantalum), or metal oxides (such as thorium dioxide, zirconium dioxide, chromite) or metal carbides (such as boron carbide or silicon carbide), which at the required temperatures, e.g., about 1,600 C, and above are stable, i.e., do not soften or do not oxidise in the given atmosphere.

It is obvious that the layer of active substance can be constituted either by grains or by differently shaped bodies made of said active substance or of another substance coated with said active substance.

The homogenous mixture of fuel and xidiser, having a temperature lower than its ignition temperature, is brought into contact with the surface of the layer, through which it passes with a'speed exceeding the rate of flame propagation. The active substance, placed in the'reaction space in the interior of the apparatus, fulfills in addition to the task of a burner simultaneously the function of a central radiation emitter and the heat transmission is to all practical purposes terminated already in the reaction space with a high degree of efficiency.

The apparatus embodying the invention can be provided with a reaction space consisting of a single chamber. According to a further embodiment of the invention a plurality of basic operative members, i.e., partial or independent reaction spaces, may be used instead of said single chamber reaction space; said basic operative members are adapted to work either independently or are grouped to a battery,-which-is submerged into the liquid to be heated. The metal walls enclosing the basic operative member are substantially the only heat exchange surfaces in the apparatus. The net cross-section of the individual operative members is chosen such as to ensure a stabilised, equally high pyrometric (operative) temperature at the surface of the grains or bodies forming the active layer, as in a single-chamber reaction space and, furthermore to ensure that any one of the isothermic surfaces inthe active layer coincides at a given point with the plane of the cross-section through the reaction space or operative member.

This means that the same temperatures prevailing at any point of the reaction space are present in a planar and not in a spatial isothermic surface and that such isothermic surface is perpendicular to the main axis of the reaction space.

The fulfilment of the first condition ensures that radiation will be effected in the most effective wavelength range. The fulfilment of the second condition prevents the formation of an inefficient or dead core of highest temperatures in the active layer in a given cross-section and, at the same time, prevents any unnecessary increase in the height of the operative member. Considering that the height of the basic operative member depends on the chosen temperature of the outflowing combustion products and is constant for a given case, there cannot exist but a single size of the cross-section area of the basic operative member, which meets both requirements and represents, at the same time, the smallest possible cross-sectional area under optimum conditions. The maximum periphery of said smallest cross-sectional area, which is also the periphery of the operative member, subjected to radiation, is however limited by the dimension of the shortest side of the cross-section.

By assembling the operative members, having optimum areas of net cross-section, to a battery, it is possible to obtain ahighly advantageous ratio, between the size of the heat exchange surfaces and the total volume of the reaction space. This results in a desirable miniaturization of the apparatus, in particular where higher outputs are concerned. In this event the basic operative members are assembled in parallel relationship. The pressure losses remain constant and are not increased with growing outputs, because the height of the active layer remains constant and the apparatus need not be equipped with further heat exchange surfaces for heat transmission by convection. Such further heat exchange surfaces would lead to an increase of pressure losses.

The arrangement of the basic operative members or their assembly according to this embodiment of the invention makes it possible to produce the gas flow through the apparatus by underpressure instead of overpressure. In this alternative embodiment of the new apparatus the system of distribution nozzles or slots, feeding the fuel-air mixture to the reaction space, may be dispensed with so as to further reduce the pressure losses. The necessary auxiliary instruments, serving for the control and upkeep of the combustion process, are thereby considerably simplified.

BRIEF DESCRIPTICN OF THE DRAWINGS Two exemplary embidoments of the new apparatus are represented diagrammatically in the accompanying drawings, showing a boiler for central heating of apartments.

FIG. 1 shows a vertical section through the boiler,

FIG. 2 is a horizontal section through the boiler in a plane passing above the water inlet,

FIG. 3 is a vertical section through the boiler along a plane parallel to the short side of the boiler,

FIG. 4 is a sectional view in elevation, showing a heating battery in a modified embodiment of the apparatus, and

FIG. 5 is a horizontal section showing the same battery in a plan view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS, 1, 2 and 3 the boiler according to the first embodiment of the invention, comprises an inner jacket 1 whose plan view has the shape of an elongated rectangle; alternatively, this plan view can have a rounded shape or the shape of an elongated ellipse. The operative surface of the inner jacket 1 is enlarged by ribs 2. The inner jacket 1 is connected to an outer jacket 6 of the boiler by means of flues 5 and at the bottom equipped with a supply conduit 3 for the admission of the prepared fuel mixture. The supply conduit 3 connects a not-illustrated fuel-air mixer to the inner space of the boiler, hereinafter called reaction space 1.3, over a system of nozzles or slots 4. The nozzles or slots 4 may be of various shapes, it being important that they should distribute the mixture uniformly over the entire profile of the reaction space 13 and, as far as possible, prevent the mixture flowing around the heat exchange surfaces, i.e., the walls of the reaction space 13.

The space between the inner jacket I and the outer jacket 6 of the boiler is filled with water 8, which is to be heated and which is fed to the boiler through a supply tube 7 and removed from the boiler through a discharge tube 9. The reaction space 13 of the boiler, in the example shown, is filled with a gas permeable layer 10 of grains, constituted by an active substance, such as zirconium silicate. The boiler is closed at its outside by a shell ll forming flues for the outgoing smoke and at its rear is equipped with a chimney 12 for the outfiowing gases.

The heating of water according to the new method proceeds in such a way that during stabilized thermal conditions a homogeneous fuel-air mixture is supplied from a not illustrated mixer through the supply tube 3 and system of nozzles or slots 4 into the reaction space 13 of the boiler, filled with the grain layer 10. The dimensions and shapes of the grains forming the layer 10, as well as the combustion process itself are chosen in relation to the volume of the boiler and the required height of the combustion zone, as will be explained hereinafter. This results in a stabilization of the dynamic equilibrium between the flow of the fuel mixture and the speed of the combustion progress, and the thermal zones are divided substantially according to the planes A A, B B, C C, D D as shown in FIG. 3. The total height of the layer 10 extends, according to this Figure, from the plane A A do the plane D D. The part of the layer 10 between the planes A A and B B remains cool. In the part of the layer between the planes B B to C C a complete combustion of the fuel mixture takes place, the temperature of the layer 10 being higher by several hundreds of centigrades than the temperature of gases between the grains of the layer 10. The metal surfaces of the inner jacket 1 and ribs 2 do not interfere with the combustion process in this layer.

About half of the volume of the layer 10 participates in the combustion and this is sufficient, since 1 dm of the layer 10 releases 30,000 to 100,000 kcal/hour. At this concentration of released heat, when the pyrometric temperatures of the layer 10 is markedly shifted towards the theoretical combustion temperature of the given fuel, even during a fairly high rate of heat removal, the known complex changes in the interior of the atomic structure of grains in the layer are taking place, whose result is a continuous conversion of released heat into radiation energy, lying primarily in the infrared part of the spectrum and according to the temperature attained, extending as far as its visible portion, i.e., mainly in the wavelength range between 6 to 0.5 micrometers.

By a suitable correlation between the manner in which the combustion process is conducted, the temperatures attained and the character of the active substance, the operational width of that part of the spectrum which is of decisive importance for the heat transmission according to the invention, is narrowed to the aforementioned wavelengths.

The radiation energy impinges in this section on the inner jacket 1 and ribs 2 of the boiler and is, without any'losses, converted into heat, which is transmitted to the water 8 to be heated. in the plane C C (FIG. 3) the temperature of gases begins to exceed the temperature of grains in the layer 10 and from said plane C C to the plane D D, heat transmission by radiation and convection of combustion gases onto the relatively large surface of grains in the layer 110 takes place. This part of the layer 10 is thereby heated and, by radiation within the range of large wavelengths,'i.e., about 6 micrometers, it transmits heat to the inner jacket 1 of the boiler. The gas temperature at the end of the layer 10 in the plane D D, under full thermal load of the boiler, becomes stabilized according to the total height of the layer 10 between 200 300 C and gases of this temperature leave through the flue 5 into the space between the outer jacket 6 and the shell 11 of the boiler, whence they are transferred through the duct i2 into the chimney at a temperature under 200 C, after they have transmitted a part of their heat to the water 8 IlII'OLlgll the outer jacket 6 and another part of their heat through the shell 11 into the space housing the boiler. It will be understood that in view of the results obtained with a boiler according to the example quoted hereinafter, the shell ill can be omitted, without affecting the practical performance and economy of operation of the boiler.

The heat transmission, effected under the described conditions, enables a reduction in the size of the heating surfaces to less than 10 percent as compared with known boiler designs an thereby a reduction of weight of the boiler according to the invention to less than 10 percent in comparison with known boilersThe combustion proceeds perfectly with an excess of air of about 3 percent.

FIGS. d and 5 show a modified embodiment of the new apparatus, in which the reaction space is built up of a plurality of basic operative members in the shape of a battery.

The heating battery shown in these Figs. comprises four basic operative members 21 of square crosssection, each of their four walls being surrounded by water 22 which has to be heated and flows in a space enclosed between the members 21 and anouter jacket 23. Each operative member 2l is filled with a layer 24 of the above described type, which rests on a grate 25 in the lowerpart of the member 21. Heating gas is supplied in the direction of arrows 25 and mixed with air, admitted in the direction of arrows 28, in a mixer ring 29, whence it is fed into a feeding chamber 26. The

combustion products are withdrawn through a common conduit 30 into an exhaustor not shown in the drawing. An ignition opening 31 leadsinto the feeding chamber 26. Cool water 22 is admitted through a tube 32 and heated water is withdrawn through a discharge tube 33. i

In the illustrated heating battery, when in thermally stabilized state, the water is heated in the following way: The not illustated exhaustor withdraws the combustion gases through the conduit 30, producing thereby an underpressure or vacuum in the entire apparatus. Consequently, not preheated combustion air 28 enters the feeding chamber 26 and mixes with the cool gaseous fuel 27. The combustile mixture is homogenized in the mixer ring 29 and flows through the feeding chamber 26 and openings in the grate 25, into all basic operative members 21. In this way the gas flows between the grains of the layer 24 with a speed exceeding the rate of flame propagation in the combustible mixture. Due to this speed, the grains of the layer 24 lying immediately on the grate 25 remain cool, protecting thereby the grate 25 from overheating on the one hand, and the combustion mixture from burning in the feeding chamber 26, on the other hand. In the lower half of the layer 24 in each of the four basic operative members 21 an intense and; perfect combustion of the combustible mixture under very high temperatures at the surface of grains in the layer 24 takes place, said layer transmitting substantially the total heat, released by combustion, by radiation onto the peripheral heat exchanging walls of the operative members 21, which pass it on to the water 22.

in order to prevent the formation of an ineffective or dead core of high temperatures in the layer 24, which would be unable to transmit the released heat directly to the heat exchanging walls and would have to transfer it to the higher parts of the layer, which would in an undesirable way increase the height of the layer 24 and, at the same time, the height of the operative members, it is necessary to maintain for a given case the net square cross-section of the basic operative members 21, each side having a lengthof about 50 mm, which corresponds to a size of grains in the layer 24 amounting toabout l0 15 mm. Under these conditions the height of the layer 24., depending on the type of the gaseous fuel employed, will amount to about 200 to 280 mm. The thermal output of each basic operative member 21, under these conditions, will have a value of about 12,000 kcal/h. The combustion gases will reach a temperature of about to 300 C in the common discharge conduit 30, provided the height of the layer 24 is chosen in accordance with the above conditions. i

As has been mentioned before, the pyrometric temperature in the part of the reaction space where combustion occurs, is maintained at a value at which substantially the complete heat transmission by radiation takes place already in the reaction space. If this value were exceeded, it would result in an increased temperature of the outflowing gases and introduction of convection surfaces, which, to all practical purposes, would represent a step backwards to the existing devices. lf this value is not reached, the output will be reduced but radiation will remain.

The smallest and simultaneously optimum size of the cross-sectional area of the basic operative member in the embodiment shown in FIGS. 4 and 5 is attained, if

any isothermic surface of the layer 24 represents a planar and not a spatial surface and if it coincides with a plane perpendicular to the axis of the reaction space 13.

The various operative members 21 represent the smallest operative unti, which if larger outputs are required are preferably grouped to batteries. In the embodiment shown in the drawing, the gas flow through the operative member 21 is produced by underpre ssure (vacuum) instead of by overpressure, as in the example shown in FIGS. 1, 2, 3 and the mixer ring 29 as well as the feeding chamber 26 are common to all operative members 21. The nozzles or slots 4 serving for the supply of the combustible mixture into the reaction space, as used in the embodiment according to FIGS. 1 3,are replaced, as shown in FIGS. 4 and 5, by a grate 25.

On connection with both embodiments, however, it is important to note that, if optimum conditions are to be achieved, the shortest distance of the peripheral or inner jacket of the reaction space from a plane passing through the longer axis of symmetry of the crosssectional profile should equal the length, at which the temperature in the axis of symmetry of the reaction space does not yet exceed the temperature at any point of the cross-section through the reaction space or operative member.

in the foregoing disclosure the use of a gaseous fuel was presumed. it is, however, equally possible to use any kind of liquid fuel, which can be transformed by evaporation into gaseous state or which in a finely dispersed suspension can easily be ignited.

EXAMPLE For a better understanding of the invention reference will be made to the following example relating to a hot water boiler as used for central heating and illustrated in FIGS. 1 to 3. i

The boiler is set in operation by setting the supply of the combustible fuel mixture to about one fifteenth to one twentieth part of the nominal boiler output. The mixture then flows between the grains of the layer at a speed of approximately 0.5m/sec. At this speed the mixture is ignited by suitable electrical means closely above the nozzles 4. The mixture burns with a visible flame and heats the lowest portion of the layer 10. After about seconds the amount of supplied mixture is increased. The flames gradually disappear and the surface of the active substance layer is heated to white heat. The stabilised state of the boiler is reached within about 60 to 90 seconds from the moment of igniting the mixture.

The boiler operates with the following parameters:

Fuel used: propane-butane mixture(% propane, 80% butane) ignition temperature of mixture 680C Temperature of admitted mixture: 20C

Speed of mixture between the grains of layer 10:

Specific load of inner heating surface: 165.500 kcal/m.h

Amount of heat transmitted by outer heating With a view to the small dimensions, low weight, high efficiency, perfect combustion and quick achievement of full performance (within about 60 seconds), as achieved by the new apparatus, the invention is suited also for the miniaturization of large and medium heating installations, for housing estates or urban units, where the apparatus due to its small weight can even be placed on the roof of houses. A high efficiency of the apparatus is achieved and problems arising from aggressive condensates exhalated by chimneys are eliminated to a considerable degree.

The invention can further be employed for heating of trains, of temporary workplaces on building sites and the like, by means of easily displaceable heating appliances, further for boilers in industry and power engineering after a suitable adaptation of the heat exchange surfaces to withstand pressure, for package boiler, thermochemical installations in oil industry, thermochemical installations for heating and evaporation of liquids in industrial plants, in organic and inorganic chemistry, in the motor-car industry for introducing steam traction based on water or other liquids, in combination with a miniature turbine with regard to the high requirements for perfect combustion and protection of environment.

High-quality fuels, in water-heating apparatus serving for heating purposes according to the invention, are burnt with an efficiency exceeding by 10 and more percent the efficiency of known high-class boilers of classic conception.

We claim:

1. A method of heating liquids, their evaporation, distillation, thermal decomposition, synthesis and the like, in which into a reaction space a perfectly homogeneous mixture of gaseous fuel and oxidizing agent is introduced, comprising feeding a homogenous mixture of gas and oxidizing agent at ambient temperature into a layer of a gas permeable radiation substance, gradually igniting such mixture in this layer with a time lag caused by a higher flow velocity of the mixture between the grains of the radiation substance then the rate of flame propagation in the mixture, said layer of radiation substance being uniform in its full width and height, and cooling such layer at its periphery only, whereby the radiation substance is brought to a condition in which the entire thermal energy, released at its surface, is transmitted by radiation in the wave length range of 0.5 to 6 micrometers to heat exchange surfaces with the exception of the heat entrained by the combustion products from said layer at a temperature below 240 C.

2. A method as in claim 1 wherein the pyrometric temperature in that part of the gas permeable layer of active substance, in which the combustion takes place, is kept at a value, at which a substantially complete heat transmission by radiation occurs already in the layer of active substance.

3. A method as in claim 1, wherein said active substance is constituted of zirconium silicate.

4. A method as in claim 1, wherein said active substance is constituted of a material, selected from the group Consisting of metals, metal oxides, and metal carbides, which are stable in the given atmosphere at temperatures about 1,600 C and above, without softening or oxidising.

5. A method as in claim 1, wherein said active substance is constituted of a material capable of selective radiation, preponderantly in the selected part of the spectrum.

6. An apparatus for heating liquids, their evaporation, distillation, thermal decomposition, synthesis, and the like, comprising in combination an inner jacket, and an outer jacket, spaced therefrom, the space between both jackets being adapted to hold the liquid to be heated, means for supplying the liquid into, and discharging it from said space between the two jackets, a reaction space in the interior of the inner jacket, wherein the cross-sectional area of said reaction space in any plane perpendicular to the main axis of the reaction space is dimensioned such that each crosssectional area is an isothermic surface, while the parallel isothermic surfaces at various points in the direction of the main axis to the reaction space have different temperatures, a gas permeable layer of a radiating substance, adapted to produce a thermal condition for radiating substantially the entire thermal energy, released at its surface, in the wave length range of 0.5 to 6 micrometers, provided in said reaction space, means for supplying a homogenous mixture of fuel and oxidizing agent, having a lower temperature than its ignition temperature, with a speed exceeding its rate of flame propagation, through said gas permeable layer in the reaction space and means for withdrawing the combustion products from the reaction space.

7. An apparatus as in claim 6, comprising further a mixing device for preparing a homogeneous mixture of fuel and oxidising agent and a system of distribution nozzles interposed between the mixing device and said layer of active substance.

8. An apparatus as in claim 6, wherein said reactor space is divided into a plurality of operative members,

wherein said reaction space is divided into a plurality of operative members, submerged in the liquid to be heated, the metal walls of said operative members being substantially the only heat exchange surfaces, the individual operative members representing the smallest operative unit, which for larger outputs are grouped in batteries.

9. An apparatus as in claim 6, wherein said reaction space is divided into a plurality of operative members, and submerged in the liquid to be heated, the metal walls of said operative members being substantially the only heat exchange surfaces, and comprising a rate to support the layer of radiating substance and to admit the homogeneous mixture into the reaction space. 

1. A method of heating liquids, their evaporation, distillation, thermal decomposition, synthesis and the like, in which into a reaction space a perfectly homogeneous mixture of gaseous fuel and oxidizing agent is introduced, comprising feeding a homogenous mixture of gas and oxidizing agent at ambient temperature into a layer of a gas permeable radiation substance, gradually igniting such mixture in this layer with a time lag caused by a higher flow velocity of the mixture between the grains of the radiation substance then the rate of flame propagation in the mixture, said layer of radiation substance being uniform in its full width and height, and cooling such layer at its periphery only, whereby the radiation substancE is brought to a condition in which the entire thermal energy, released at its surface, is transmitted by radiation in the wave length range of 0.5 to 6 micrometers to heat exchange surfaces with the exception of the heat entrained by the combustion products from said layer at a temperature below 240* C.
 2. A method as in claim 1 wherein the pyrometric temperature in that part of the gas permeable layer of active substance, in which the combustion takes place, is kept at a value, at which a substantially complete heat transmission by radiation occurs already in the layer of active substance.
 3. A method as in claim 1, wherein said active substance is constituted of zirconium silicate.
 4. A method as in claim 1, wherein said active substance is constituted of a material, selected from the group consisting of metals, metal oxides, and metal carbides, which are stable in the given atmosphere at temperatures about 1,600 * C and above, without softening or oxidising.
 5. A method as in claim 1, wherein said active substance is constituted of a material capable of selective radiation, preponderantly in the selected part of the spectrum.
 6. An apparatus for heating liquids, their evaporation, distillation, thermal decomposition, synthesis, and the like, comprising in combination an inner jacket, and an outer jacket, spaced therefrom, the space between both jackets being adapted to hold the liquid to be heated, means for supplying the liquid into, and discharging it from said space between the two jackets, a reaction space in the interior of the inner jacket, wherein the cross-sectional area of said reaction space in any plane perpendicular to the main axis of the reaction space is dimensioned such that each cross-sectional area is an isothermic surface, while the parallel isothermic surfaces at various points in the direction of the main axis to the reaction space have different temperatures, a gas permeable layer of a radiating substance, adapted to produce a thermal condition for radiating substantially the entire thermal energy, released at its surface, in the wave length range of 0.5 to 6 micrometers, provided in said reaction space, means for supplying a homogenous mixture of fuel and oxidizing agent, having a lower temperature than its ignition temperature, with a speed exceeding its rate of flame propagation, through said gas permeable layer in the reaction space and means for withdrawing the combustion products from the reaction space.
 7. An apparatus as in claim 6, comprising further a mixing device for preparing a homogeneous mixture of fuel and oxidising agent and a system of distribution nozzles interposed between the mixing device and said layer of active substance.
 8. An apparatus as in claim 6, wherein said reactor space is divided into a plurality of operative members, wherein said reaction space is divided into a plurality of operative members, submerged in the liquid to be heated, the metal walls of said operative members being substantially the only heat exchange surfaces, the individual operative members representing the smallest operative unit, which for larger outputs are grouped in batteries.
 9. An apparatus as in claim 6, wherein said reaction space is divided into a plurality of operative members, and submerged in the liquid to be heated, the metal walls of said operative members being substantially the only heat exchange surfaces, and comprising a rate to support the layer of radiating substance and to admit the homogeneous mixture into the reaction space. 